With the help of sitemap you can tell the google about your pages inorder to crawl these pages. When you make a change in any page of your website, then you have to update the sitemap and resubmit it to google so that it crawl the page .
sitemaps make google's job easeir but it doesnt really affect your ranking much... it does however notify google of any pages that it might have missed
Sitemap is important to point out search engine that they can find all the website links at one place and they can easily crawl them.